fix ZipkinExporter when using BackgroundThreadTransport (#247)
fix ZipkinExporter when using BackgroundThreadTransport (#247)

* changed time precision on zipkin exporter to microseconds

* fixed tests

* fixed line length

* fixed translate_to_zipkin function on ZipkinExporter
- previously when using BackgroundThreadTransport, all traces were sent with same trace_id

census-instrumentation/opencensus-python,census-instrumentation/opencensus-python,census-instrumentation/opencensus-python